This invention relates to a video memory device for use in a special effect device which can accomplish a special effect on a television screen by processing a television signal of a succession of picture elements.
A conventional video memory device of the type described can carry out write-in and read-out operations of a succession of picture elements subjected to pulse code modulation (PCM). With the video memory device, it is possible to accomplish various kinds of special effects on a television screen by modifying either one of a write-in speed and a read-out speed from the other. For instance, let the read-out speed be reduced to a half of the write-in speed. In this event, each picture element which is derived from an original image is slowly read out of the video memory device and can form a reproduced image which has twice as wide an area as the original image. However, such a reproduced image inevitably has a degraded picture quality as compared with the original image because each picture element is visually reproduced two times on the reproduced image.
In order to avoid the degradation of picture quality, interpolation is often carried out between two adjacent ones of the picture elements that are derived from the picture element succession. In other words, two adjacent picture elements are necessary for the interpolation. The video memory device is therefore accessed two times on carrying out the interpolation. When all of the picture elements are read out of the video memory device to be reproduced on the television screen, the interpolation can comparatively readily be carried out by the use of the two adjacent picture elements.
However, all of the special effects do not always necessitate reproductions of all picture elements. For example, a certain one of the special effects needs to reproduce intermittent ones of the picture element succession and interpolated picture elements. In order to obtain the interpolated picture elements, two adjacent ones of the picture elements should be read out of the video memory device as mentioned before even when they are not visually displayed on the television screen.
At any rate, the conventional video memory device must sequentially be accessed twice on carrying out the interpolation. Therefore, the interpolation is time-consuming with the conventional video memory device.